DE 38 36 660 C discloses a heat-sensitive recording material of the type described above. The back coating can be formed from water-soluble high polymers, such as starch, gelatin, styrene-maleic anhydride copolymer hydrolyzates, or polyvinyl alcohol, and from water-insoluble polymers, such as latices. In this connection, the polymers are used alone or as a mixture. When the previously known recording material is used as a label, the previously known back coating is intended to prevent any plasticizers in particular which may be present on the sheet that is to be provided with the label from penetrating into the recording layer from the back and thus causing the printing which may be present from fading or disappearing.
DE 37 20 171 A (CA 1,283,780 C) hopes to realize a comparable protective effect for the barrier layer applied to the back of a proposed heat-sensitive recording material that has a colored fluorescent dye composition in its recording layer. The barrier layer comprises essentially water-soluble polymer materials, such as polyvinyl alcohol, various cellulose ethers, starch, gelatin, casein, and polyvinyl pyrrolidone, and water-dispersed resins such as polystyrene emulsions. The function of the barrier layer is protection from materials such as oils and plasticizers known to be able to cause discoloration and/or decoloration of the images developed in the recording layer.
The objective of WO 99/14056 A1 (U.S. Pat. No. 6,326,330 B1) is to allow back-side printing of a heat-sensitive recording material by offset printing or flexographic printing after the application of a back coating that has a good barrier effect against the substances used in offset and flexographic printing, especially organic solvents, as well as against plasticizers, oils, and fats. To achieve this objective, the back coating of the previously known recording material has a mixture containing starch, an acrylate copolymer which is free of styrene or vinyl acetate components and has a film-forming temperature below 5° C., preferably below 2° C.; and an alkaline catalyst such as calcium carbonate.
Thus, it is precisely this last-cited document which reveals that the back of heat-sensitive recording materials can be used as a substrate for offset printing. The prior-art recording materials have absolutely proven themselves effective with respect to their barrier effect against the chemicals used with them. If, on the other hand, high-grade, pigment-containing back coatings are desired for, for example, full-surface reproduction of multicolor prints, in the past it was necessary in some cases to deal with considerable difficulties in the actual printing process with its demands on the surface to be printed, because deposits on the blanket make trouble-free and thus effective printing more difficult. Therefore, the object of the present invention is to make available to the public a heat-sensitive recording material with a back coating which offers a barrier effect against substances used in offset and flexographic printing, especially organic solvents, plasticizers, oils, and fats, and which at the same time can be printed well without any problems by the offset and flexographic printing processes.